Method of producing hexafluorophosphoric acid



Patented Nov. 15, 1949 n sitar METHOD:- OF PRODUCIIJG, HEXAFLUOR--PHOSJEHORIC. ACID of- Delaware No Drawing, Application Decemb'er'd,1947, SerialNo. 789,758. In Canadaduly 1'7, 1947 4 Claims. 1

This:v invention: relates: to the production ofhexafluorophosphoricacid; HBFe; hereinafter'for: brevity termed1hexaacid iin accordance with. the method. originallydisclosed in*ouncopending. application Serial No. 696,620; filed September 12, 1946,entitlediMethcd off producing and purifying hexafiuorophosphoric acid ofwhich this: is a; division;

In the said'application wehave: described ourmethod offmaking. hexaacidby reaction betweenhydrogen fluoride and. certain compounds ofpentavalentr phosphorus and have claimed the said method as it may bepractised by use ofoxygen containing. compounds of pentavalentphosphor-usywhichionrreactionrwith hydrogen fluoride under theconditions specified in the said application yield oxygen-to combinewith; the hydrogen of the hydrogen fluoride and produce water serving asa-n-ionizationmedium for the hexa acid which cannot exist inanhydrous-state.

Also disclosed in said application is our method of producing hexa acidfrom, a compound of pentavalent phosphorus devoid of oxygen,f.or,exsample PFs; whichis incapable" ofproducing' in anhydrousreaction-with'HF thewater needed as an ionization medium for hexaacid,and thisdivisional application is directed specifically -to;this aspectof" our" invention: whicl'r is" appropriately: claimed herein.-

It is thus an object of the present invention to provide pursuant to theprinciples set forth in said prior copending application a method forproducing hexa acid from a compound of pentavalent phosphorus devoid ofoxygen, specifical= ly PF's.

Other objects, purposes and advantages will appear as the followingdescription proceeds, it being understood the disclosure in said priorcopending application of certain broad general principles relating tothe behavior of phosphorus compounds in the presence of HF may be con--sidered as incorporated herein by reference.

As more fully explained therein, hexa acid is incapable of independentexistence in the anhydrous state, but may be produced in a hydroussystem in which it may exist in equilibrium such that when the solutionis heated under conditions preventing the loss of volatile and/orgaseous constituents and then cooled to the original temperature orbelow no quantitative loss of hexa acid occurs. At constant temperaturein equilibrium solution the hexa acid when present in fairly highconcentration does not hydrolyze; as dilution is increased by additionof water hydrolysis takes place at a decreasing rate until at aconcentration of a few per cent HPFslit becomes-imperaceptible and afalse appearance of? stability-is reached. If, however; thedilutesolution.issheatted in a closed system the HPFG' de'composesrand:is not reconstituted on subsequent:cooling;.show ing it actually is notstable.

To produce HPFs in equilibrium;therefore ittis necessary that certainratios of; the. quantitiesxofi the materialssupplied be adhered: to:and? that: conditions be maintained during: the reactiom suitable .for.insuring the retentionsofigasecmsand? volatile constituents; when 2E5isused' asia; start-.- ing material, beingitselt'an-gas:at.;ordinaryrteme' peratures, it isparticularly:importantrthese conditions be observed.

Itis essential also that some waterzbe present, althoughiwhethensupplied as-such .or. asdiluent. of HF appearsimmaterial',,and.it isnotlpossiblie. definitely to specify. an. absolutemini'mumi limit for.the proportioncof. HF thatlmustbe. supplied,

The, water present, however, may. notv exceed 15225. moles per mole. ofPFs, and'HF must'b'e sup.- plied'. at leastin quantity sufficienttosatisfy the ratio 1 mole HF: 1.625" moles Hi0 as otherwise. the waterseems toinhibit:reactionbetweenPFs and HF; Thus whilethere must be somewater available to supply an ionization vmedium for the HPFe,produced,,either too much water in relation to the amount of "PFsor toomuch inrelatibn to the amount of'HF'must be avoided;

Under suitable conditions, however, with. water present within thelimits stated, HFandphOsphorus pentafluoride react together to yieldhexa acid substantially in accordance with the following equation:

As noted, escape of gaseous constituents including PF5 should beavoided, either by introducing the materials separately into a closedcontainer, or by passing gaseous PF5 into a solution of HF maintained atlow temperature, such as at or below 10 C.

The latter procedure using 1.9 parts of a solution containing 52.6% HFand 6.3 parts of gaseous PFs and thus with the constituents of thereacting mass substantially conforming to the ratio 1 mole PF5 1 mole HFZ 1 mole H2O yields a solution containing 84.5% HPFe, corresponding to94.9% of the theoretical yield.

Such solution, as more fully explained in said prior application, may bepurified if desired by further dilution with water followed by coolingto precipitate hexafiuorophosphoric acid hexahydrate, recovering thecrystalline precipitate and redissolving it in water to produce a puresolution of HPFs suitable for use in industry and the arts.

It will be appreciated that for best yields of HPFe it is desirable tosupply an excess of HF above that required to just equal the sum of 1mole HF 1 mole PF5 and 1 mole HF 1.626 moles H2O, in other words toemploy aqueous HF oi" a concentration of at least about 40% HF, as amoderate excess of HF tends to drive the foregoing equation to the rightand thus increase the yield of HPFs but larger amounts have yielded noproducts indicating a higher degree of fiuorination than thatrepresented by the formula HPFG can be attained.

The stated maximum limit for water present, namely 16.25 moles 1 molePF5 may not be exceeded, however, as beyond this limit no amount of HF,however large, appears to bring about equilibrium formation of HPFe fromPF5, while the minimum limit for water is determined, not by thereadiness of the reaction between HF and PFs to proceed, but by thenecessity for an ionization medium for the product of the reaction whichcannot occur in a wholly anhydrous system but does proceedsatisfactorily with only a small amount of water present.

As water does however have some tendency to hamper the progress of thereaction this tendency may be counteracted by supplying some HF inaddition to 1 mole I-IF 1 mole P and 1 mole HF 1.625 moles H2O; forpractical purposes desirable minimum proportions for HP and H20 in thestarting materials may therefore be as indicated by the followingequation:

in which a: may be any number significantly greater than and not greaterthan 16.25.

While we have thus particularly described the method of this inventionwith reference to the production of hexa acid from PF it will beunderstood we do not desire or intend to limit or confine ourselvesthereto as changes and modifications therein which will occur to thoseskilled in the art may be made, without departing from the spirit andscope of the invention as defined in the appended claims.

Having thus described our invention, we claim and desire to protect byLetters Patent of the United States:

1. The method of producing hexafiuorophosphoric acid which comprisesmixing phosphorus pentafiuoride with water-containing hydrogen fluoridein which hydrogen fluoride is present in excess of 1 mole per 1.625moles of water in the ratio of about 1 mole of hydrogen fluoride permole of phosphorus pentafiuoride, and preventing escape of gaseouscompounds during the reaction until equilibrium is attained.

2. The method of producing hexafiuorophosphoric acid which comprisesmixing phosphorus pentafiuoride with hydrogen fluoride in the presenceof water not in excess of 16.25 moles of water per mole of phosphoruspentafiuoride in the ratio of about 1 mole of hydrogen fluoride per moleof phosphorus pentafiuoride, and preventing escape of gaseous compoundsduring the reaction.

3. The method of producing hexafiuorophosphoric acid which comprisesprogressively introducing gaseous phosphorus pentafiuoride into a watersolution of hydrogen fluoride containing in excess of 1 mole of hydrogenfluoride per 1.625 moles of water until more than 1 mole of phosphoruspentafiuoride per 16.25 moles of water present has been introducedthereinto while maintaining the solution at a reduced temperature tothereby inhibit escape of gaseous compounds therefrom until equilibriumis attained.

4. The method of producing hexafiuorophosphoric acid which comprisesprogressively introducing gaseous phosphorus pentafiuoride into a watersolution containing at least about 40% by weight of hydrogen fluorideuntil equilibrium isattained. Y

WILLY LANGE. RALPH LIVINGSTON.

REFERENCES CITED The following references are of record in the OTHERREFERENCES Mellor, Comprehensive Treatise on Inorganic and TheoreticalChemistry, vol. VIII, pages 998-(1928) Longmans Green & Co.

